1. Field of the Invention
The present invention relates to a hot runner sprue.
2. Description of the Related Art
Use of a hot runner, as in an injection molding mold 10 for thermoplastic resin shown in FIG. 14, allows a manifold 12 and a nozzle 13 to be heated and resin 14 to be held constantly in a molten state. Further, utilization of the hot runner allows the molded part to be ejected from the injection molding mold 10 while only chilling a part molding cavity 16. The injection molding mold 10 is provided with a hot runner block 18 that ensures a flow path for the resin 14 at the time of injection. Note that, the element indicated by reference numeral 20 in FIG. 14 is a heater (for an example, refer to Japanese Patent Application Publication No. JP-A-4-67923 (Pages 1 and 2, and FIGS. 1 and 2)).
A standard valve gate hot runner block 22, as shown in FIG. 15, normally has a structure in which the supply of resin into the part molding cavity is controlled by blocking a gate 24 of the nozzle 13 using a stem 26. However, with this structure, when the molding machine nozzle is separated from the nozzle touch unit 28a of the hot runner sprue 28 to change the mold or the like, resin in the manifold 12 and the hot runner sprue 28 returns and leaks from the nozzle touch unit 28a. As a result, the pressure within the manifold 12 and the hot runner sprue 28 drops down toward atmospheric pressure. Further, air enters into the hot runner sprue 28 and the manifold 12 from the nozzle touch unit 28a and forms air pockets 32 in the runner 30. The next time molding is carried out, these air pockets 32 cause silver streaks (one type of molding defect) in the molded part. Such silver streaks will continue being formed in the molded parts until all of the resin within the nozzle 13, the manifold 12, and the hot runner sprue 28 has been ejected from the gate 24, and completely replaced with resin containing no air pockets. As a result, in the case of molded parts with a small production quantity for each lot, the reject rate will increase.